Updated August 6, 2026. Quick answer: they are not the same window and they do not let you do the same things. October 15 to December 7 is the one where almost anything is possible. January 1 to March 31 is only open to people already in a Medicare Advantage plan, and it lets them do exactly two things. If you are in Original Medicare in February and want to change your drug plan, that window is not for you.
The two windows, from Medicare’s own table
Open Enrollment Period, October 15 – December 7. Medicare.gov lists what it allows: “Join, drop, or switch to another Medicare Advantage Plan with or without drug coverage (or add or drop drug coverage). Switch from Original Medicare to a Medicare Advantage Plan or from a Medicare Advantage Plan to Original Medicare. Join, drop, or switch to another Medicare drug plan if you’re in Original Medicare.” Coverage starts “January 1 of the next year (the plan must get your enrollment request by December 7)”.
Medicare Advantage Open Enrollment Period, January 1 – March 31 — and the qualifier is in the name of the row itself: “only if you’re already in a Medicare Advantage Plan”. What it allows: “Switch to another Medicare Advantage Plan with or without drug coverage. Drop your Medicare Advantage Plan and return to Original Medicare. You’ll also be able to join a separate Medicare drug plan.” Coverage starts “first of the month after the plan gets your request”.
What the second window does not do
Read the two lists side by side and the gaps are the whole point.
You cannot move from Original Medicare into a Medicare Advantage plan in the January window. That action appears only in the October list. If you are in Original Medicare, the January–March window does not exist for you.
You cannot switch a standalone drug plan in the January window while staying in Original Medicare. The drug-plan permission in that row is attached to leaving a Medicare Advantage plan — you join a drug plan because you are returning to Original Medicare, not as a change in its own right.
And neither window is a Medigap window. This is the expensive one. Dropping Medicare Advantage in March and returning to Original Medicare does not entitle you to a Medigap policy at any price — outside a guaranteed-issue situation, a Medigap insurer can medically underwrite you and decline. The underwriting window is the rule that decides it, trial rights are the exception, and a handful of states give you an annual right your neighbours do not have. Anyone who tells you that you can “always switch back” is describing Medicare Advantage, not Medigap.
So which one do you actually want?
Use October–December for anything structural. Changing between Original Medicare and Advantage in either direction, changing a standalone Part D plan, adding drug coverage. Coverage starts January 1, and the deadline is the plan receiving the request — not you posting it — by December 7. The checklist for that window is the one to work through.
Use January–March as a correction, not a plan. It exists because people discover in January that the plan they chose in October does not cover their doctor or their drug. That is what it is good for: a network that turned out to be narrower than it looked, or prior-authorisation friction you did not expect.
Everything else — a move, losing employer coverage, a plan leaving your area, qualifying for Medicaid or Extra Help — runs on a Special Enrollment Period instead, which is its own set of rules and timings.
Sources, and what we do not assert
Every quoted line is from medicare.gov’s own enrollment-period table on the “joining a plan” page, read 2026-08-06. We do not state a limit on how many times you may change plans during the January–March window: a limit is widely repeated, and medicare.gov’s table does not state one, so we are not going to publish a rule we have not read. This page exists because that kind of blurring is what makes the two windows confusing in the first place. See the Medicare glossary, methodology and corrections. General information, not advice about your plan; and no product on this page is sold, recommended, or paid for.